1. Field of the Invention
The invention relates to apparatus and techniques for measuring the concentration of an electrolyte in a specimen.
2. Brief Description of the Prior Art
Various devices have been heretofore proposed for determining the pH value or electrolyte concentration of a specimen. A substantial drawback with prior devices has been the difficulties in calibration and inability to maintain consistent reliable readings.
Several pH testing devices were relatively simple in construction and utilized a pair of electrodes of dissimilar materials which were electrically interconnected through a meter. An example of devices of this type is shown in U.S. Pat. No. 3,161,823. Unfortunately, accurate, reliable readings were unobtainable due to various reasons. For example, inaccuracies resulted from temperature factors. Further factors affecting the reliability of such testing devices included contamination of the electrodes which resulted in variations of the readings.
Attempts at providing more accurate and reliable pH testing devices included resort to use of a standard solution of known pH value for calibration during the testing process as exemplified in U.S. Pat. No. 2,604,382. Further devices utilized an intermediate electrolyte between the electrodes and separated from the specimen by a membrane as shown in U.S. Pat. No. 3,413,209. These approaches did not meet with relative success in part due to the complicated procedures involved.
Temperature compensation of pH testing devices has been attempted through the use of compensating resistors as shown in U.S. Pat. No. 3,806,797. Generally, compensated devices were more costly to produce and did not provide a commensurate increase in accuracy or reliability of readings obtained. Further calibration procedures could not adequately compensate for the effects of gradual electrode contamination.
One approach to electrode contamination was disclosed in U.S. Pat. No. 2,454,952 and involved a cleansing procedure which included a series of washings in baths of distilled water, detergent and acetic acid, distilled water again, hydrogen peroxide solution, and the final rinsing with distilled water. Consequently, the use of such device was limited to technicians having adequate reagent supplies to perform the requisite cleansing technique, and the device was unsuitable for use by unskilled persons.
In U.S. Pat. No. 2,288,180 it was suggested that contaminated electrodes could be cleaned by sanding with emery paper, however it has been found after extensive testing that successive emery paper treatments of the same electrode surfaces did not provide an electrolytically active surface of uniform electrode activity. This was possibly due to a multitude of factors; for example, the emery paper itself could have deposited a residue on the electrode and interfered with the electrolytic activity of the electrodes.